1. Field of the Invention
The present invention relates to a knocking control apparatus of an internal combustion engine.
2. Description of the Related Art
During engine operation, the engine body continues to vibrate, but if knocking occurs, the intensity of the vibration of the engine body grows larger. Therefore, it is possible to detect the occurrence of knocking from the increase in the intensity of the vibration of the engine body. Thus, usually, a knocking sensor for generating an output voltage proportional to the strength of the vibration of the engine body is mounted on the engine body and when the strength of the vibration of the engine body exceeds a predetermined knocking determination level, that is, when the output voltage of the knocking sensor exceeds the predetermined knocking determination level, it is judged that knocking has occurred.
However, the engine body vibrates even without occurrence of knocking in this way, and the strength of the vibration of the engine body differs according to each engine as well. Further, along with the increase of the rotational speed of the engine, the strength of the vibration of the engine body grows larger. That is, even when knocking does not occur, the strength of the vibration of the engine body fluctuates by a considerable amount. Therefore, if the knocking determination level is set low, it may be judged that knocking has occurred even if it has not and if the knocking determination level is set high, it may be judged that knocking has not occurred despite it having occurred. Therefore, to reliably detect the occurrence of knocking, it is important what level the knocking determination level is set at. In this case, if it were possible to set the knocking determination level higher than the strength of the vibration of the engine body where knocking does not occur, then it would be possible to reliably detect the occurrence of knocking using the knocking determination level. Therefore, to reliably detect the occurrence of knocking, it is necessary to accurately detect the strength of the vibration of the engine body when knocking does not occur. There are two conventional methods used for finding the strength of vibration of an engine body at which knocking does not occur.
These two methods will be explained with reference to FIG. 16.
FIG. 16 shows the relationship between the magnitude of the output voltage (V) of a knocking sensor at a certain engine rotational speed, that is, the amplitude of the vibration, and the number of occurrences of vibration generated at various amplitudes, that is, the frequency of occurrence. Here, FIG. 16(A) shows the case where no knocking occurs at all, FIG. 16(B) the case where some knocking occurs, and FIG. 16(C) the case where excess knocking occurs.
As shown in FIG. 16(A), when no knocking occurs at all, the frequency of occurrence of vibration of the amplitude A is the highest and the frequency of occurrence of vibration becomes lower the further away the amplitude from the amplitude A. Therefore, below, the amplitude A is referred to as the maximum frequency point.
When knocking occurs, vibration with a large vibration amplitude occurs, so as shown in FIG. 16(B), the frequency of occurrence of large amplitude vibration rises and if knocking further occurs, as shown in FIG. 16(C), the frequency of occurrence of vibration with a large amplitude rises further. Even if knocking occurs, however, the vibration of the engine body caused by reasons other than knocking occurs in the same way as when no knocking occurs, so even if knocking occurs, the maximum frequency point A does not change, as will be understood from FIG. 16.
Therefore, one of the above two methods finds the maximum frequency point A by some method or another and uses A.multidot.k, obtained by multiplying A by k, as the knocking determination level so as to judge that knocking occurs when the vibration amplitude value exceeds this knocking determination level A.multidot.k. This method is considered to be optimal as a method for judging the occurrence of knocking.
The other of the above two methods finds the mean value of the frequency of occurrence of vibration, i.e., the center of gravity position of the area of the portion surrounded by the curve shown in FIG. 16. This mean value of the frequency of occurrence of vibration is indicated by B in FIG. 16, with B hereinafter referred to as the mean frequency value. According to this method, B.multidot.k, obtained by multiplying B by k, is used as the knocking determination level and knocking is judged to occur when the vibration amplitude exceeds this knocking determination level B.multidot.k. However, in this method, as shown in FIG. 16, when the frequency of occurrence of knocking becomes higher, the mean frequency value B gradually becomes larger and along with this the knocking determination level B.multidot.k also becomes larger, so if the frequency of occurrence of knocking becomes higher, there is the problem that it becomes impossible to detect the knocking. Therefore, as the knocking determination level, it is said to be better to use A.multidot.k than B.multidot.k.
Thus, the problem lies in how to find the maximum frequency point A. Regarding this, there is known a knocking control apparatus which finds the vibration amplitude a predetermined number of times, statistically finds the distribution of the frequency of occurrence of the vibration amplitude, and finds the maximum frequency point A from the frequency distribution found statistically (see Japanese Unexamined Patent Publication (Kokai) No. 63-253155). However, when statistically finding the distribution of frequency of occurrence in this way, the amount of data to be stored becomes greater, so a large load is placed on the RAM or else it takes a long time to find the maximum frequency point A.